The S&P 500 Just Did This for the First Time in 13 Years. Here's What History Says Happens Next.

The S&P 500 (SNPINDEX: ^GSPC) is the most widely followed stock market index in the U.S. and includes the 500 largest companies in the country. Because it contains a broad swath of American businesses, it's also considered by many to be the best overall benchmark and the most reliable gauge of overall stock market performance.

The storied index has been squarely in rally mode since it bottomed in October 2022, driven higher by waning inflation, the advance of artificial intelligence (AI), and the Federal Reserve Bank's long-awaited decision to begin its campaign of interest rate cuts. These factors have combined to create an environment that's ripe for the stock market rally to continue.

The S&P 500 just delivered its best January-through-September performance since 1997 and has now entered the third year of its current bull market run, something that hasn't happened since 2011. If history is any indicator, the current rally still has much further to go.

Fresh off the worst bear market since 2009, investors are relishing the good times -- and well they should. History shows that bull markets have more stamina and tend to last much longer than their bearish counterparts.

Since World War II, the average bull market has lasted roughly four and a half years, according to data compiled by Bespoke Investment Group. For context, that's far longer than the average bear market, which lasts roughly one year.

That said, not all bull markets are created equal. For example, the bull market that started in 1987 ran for more than 12 years, while the bull market that began in 2009 ran for 11 years. On the other end of the spectrum, the bull market that began in 2001 lasted just three months.

The current rally just completed its second full year, so -- if history holds true -- this bull market still has further to run. Of the 13 bull markets that have occurred over the past 77 years, seven have lasted three years or more, so history is on the side of the bulls.

Then there's the matter of returns. Bull markets have generated returns of 152%, on average, which bodes well for current investors. However, the market gains varied greatly, depending on the length of the rally. For example, the bull market that began in 1987 generated returns of 582%, while the one that began in 2009 returned 400%. However, the short-lived rally of 2001 -- which lasted just three months -- returned just 21%.

Generally speaking, the longer the bull market, the greater the potential returns. That holds true for the ongoing run as well. Looking back to October 2022 -- the beginning of the current market rally -- the S&P 500 has generated returns of 63%. If history holds true, the current bull market has much more to give.